Manufacture of wood fiber hardboard



B. O. STEWART MANUFACTURE OF WOOD FIBER HARDBOARD Filed July 24, 1957May 26, 1959 HIGH TEMPERATURE STEAM TREATMENT OF wooo cm s REDUCE woooCHIPS SUBSTANTIALLY 'To FIBERS FORM WETMAT OF FIBERS ON SCREENsQuEEzE-PREss To REDUCE MOISTURE CONTENT DRY m HOT AIR CONTINUOUS DRYERINSTANTANEOUS SURFACE T HEAT TREATMENT HOT PRESS TO vCONSOLIDATE INTOHARDBOARD INVENTOR.

Basil 0. Stewart ATTORNEY United States Patent Ofiice 2,888,376 PatentedMay 26, 1959 2,888,376 MANUFACTURE OF WOOD FIBER HARDBO Basil 0.Stewart, Kenmore, N.Y., assignor to National Gypsum Company, Buffalo,N.Y., a corporation of Delaware Application July 24, 1957, Serial No.673,891

Claims. (Cl. 162-163) I This invention relates to the manufacture ofheat and pressure consolidated wood-fiber hardboard. More particularly,it is directed to a process for the preparation of a dried, wet-formedmat of wood-fiber prior to the consolidation thereof by combined heatand pressure.

Themanufacture of wood-fiber hardboard consists of the formation of arelatively porous mat of woody material and the consolidation of the matby combined heat and pressure. A high-pressure steam treatment of thewoody materialprior to forming, providing to some deg'ree a conversionof the lignins and the cellulose, is commonly preferred in order toobtain, when subsequently formed and consolidated, a superior hardboard,and it is with regard to this method of hardboard manufacture that thepresent invention is concerned.

Of particular concern, one result of the steam treatmentof woodymaterial is the formation of water-soluble sugary material. Followingthe mat formation and drying, the water-solubles are, in large part,ultimately deposited on the wood fibers of the mat. The usual manner ofdrying, whether or not it is preceded by passing the mat through squeezerolls to substantially reduce .the water content, will include thesubjection of the mat to circulating, relatively dry, heated air, as ina kiln. The mat is reduced in moisture content to a bone-dry conditionor at least to a moisture content of less than 10%, preparatory toconsolidation, by heat and pressure, to a relatively thinner hardboardof a specific gravity ordinarily about 1.0 or higher.

In prior processes, generally as described above, it has been consideredessential to remove, in some manner, a substantial proportion of thewater solubles at some point in. the 'process prior to the final drying,since it is known that their presence creates a problem of the surfacesticking to the high temperature and pressure consolidating means,resulting in delaminated, unacceptable end products. This has beenaccomplished heretofore by additional washing steps prior to drying,wherein the solubles are carried away in the wash water. The washingsteps are unduly costly, and, of greater significance, create a veryserious problem of waste disposal.

It is an object of the present invention to provide a process for makinghardboard generally in accordance with the above described preferredsteps, wherein the sticking caused by the solubles is avoided, withoutrequiring the costly and undesirable additional washing steps for theirremoval, and further permitting a more rapid high temperatureconsolidation.

A further object of the invention is to provide a process of makinghardboard wherein a high percentage of all of the natural woody materialis retained and employed within the final product.

A still further object of the invention is the avoidance in themanufacture of hardboard of any substantial amount of waste disposal andthe consequent stream pollution problems, and providing instead arecovery of sludge and solubles in a form readily adaptable for furtheradvantageous use.

A still further object of the invention is to provide a process ofhardboard manufacture wherein a highly impervious-surfaced hardboard isproduced, of improved bending strength and abrasion resistance.

These and other objects and advantages will appear more fully whenconsidered in connection with the following detailed description of apreferred embodiment of the invention and the accompanying drawing inwhich a flow sheet of the process of the invention is set forth.

A preferred form of the process of the present invention, as shown inthe drawing, consists of reducing raw wood to chips of a size in theorder of A" x across the grain and with the grain. The chips are thensubjected to a high temperature steam treatment, to chemically convertsome of the ingredients therein, principally the lignins, into a formwhereby they will subsequently provide a binder material in theconsolidated board, this being in accordance with many similar priorprocesses. As is known, there is also produced by this treatment, acertain amount of water soluble matter, such as wood sugars, which havelong been recognized as undesirable, but unavoidable.

The steam treated chips are then reduced in size substantially to fibersby any known means such as by defibrators having closely opposed,oppositely rotating discs. The fibrous material, dispersed in an aqueousslurry, is then formed into mats by known screen forming means, such asa Fourdrinier machine.

The mats, still consisting of considerable water, are then squeezed,such as by press rolls, to substantially reduce the water content, andare then air dried, as in a hot air continuous dryer, to a water contentof from 0% to 10%, preferably as close to 0% as practical.

In accordance with the present invention, the surfaces of the mats aredry-heated, by conduction, convection or radiation, to temperatures inexcess of 400 F., preferably to in the order of 400 F. to 500 F., forvery short intervals of time. The time period is preferably in the orderof one second, however, it is more properly defined in accordance withthe invention, as substantially the amount of time required to convert asurface-migrated layer of wood sugar from an A stage wherein thematerial is liquid when hot, solid when cold, and generally Example I Ina preferred form of the invention, the dry mat is passed between rolls,such as calender rolls, which are heated to a sufficient temperature inexcess of 400 F. to provide the above described surface materialconversion, using roller speeds and sufiicient pressure on the rolls toeffect rapid heat transfer without producing a permanent densificationof the mat to a specific gravity above approximately 0.2 less than theultimate density of hardboard desired.

The above instantaneous high temperature and pressure surface treatmenthas been found to be capable of performance without a harmful degree ofsticking occurring, still resulting in providing a surface treated matwhich may be consolidated, by the usual relatively lengthy hightemperature, and pressure process, again without encountering a harmfuldegree of sticking.

Example II The dry mats may alternatively be pressed between heatedplatens, of suflicient temperature to substantially instantaneously heatthe mat surfaces to in excess of 400 F. The mat is compressed to effectrapid heat transfer, avoiding a densification of the mat to a specificgravity in excess of approximately 0.2 less than the ultimate densitydesired. The platens are held under compression for only a suificienttime to convert the surface wood sugar material from the A stage to theB stage, in the order of one second.

The surface treated mat is then consolidated by the usual relativelylengthy high temperature and pressure consolidation.

Example Ill Dry mats, wherein a relatively high percentage of watersolubles are present at the surface, are more satisfactorily surfacetreated in accordance with the invention by altering the methods ofExamples I and II to include a plurality of repeated surface heatingsteps. Thus, in Example I, rather than decreasing the speed of the hightemperature rollers excessively to obtain additional time for completionof conversion of a suitable degree of A stage material to B stagematerial, there is performed a plurality of passes of the mat betweenone set or successive sets of heated rollers. Similarly, in Example 11,repeated bumps of the high temperature press platens may be employed toprovide the desired surface material conversion. The surface treatedmats are then consolidated by suitable high temperature and pressure.

Example IV To employ radiated heat in accordance with the invention, thedry mats are passed between gasfired, infra- ,red heaters of sufficienttemperature, and using suitable Surface treatment of mats by heatconvection, is possible in accordance with the invention, by passing thedry mats rapidly through a high temperature hot air oven, raising themat surface temperature to in excess of 400 F., without heatingsubstantially the mat material inward of the surface, converting the Astage surface material to the B stage. by suitable high temperature andpressure.

The economic element will usually dictate, in large part, the methodmost advantageous for any given manufacturing facility.

As stated, the amount of Water-solubles present may vary and requirevariation in the preferred conditions in practicing the invention. Theamount of water or other forms of moisture present will further effectthe necessary conditions. The presence of other naturally-occurring oradded material or variation in the pH may also speed up or slow down thechange from the A stage to the B stage.

Under most all conditions which may reasonably be encountered, aconversion of the water-solubles, which have been found to migrate invery high percentage to The mat is then consolidated the mat surface,may be successfully, simply and economically obtained in accordance withthe above methods, whereby the high temperature and pressureconsolidation, to produce hardboard, may be carried out with greaterfacility than was common heretofore. As a further result, an improvedhardboard is produced for equal or less cost, having substantially allof the natural woody material remaining and beneficially employed,providing greater strength and cohesiveness, imperviousness andresistance to abrasion.

Having completed a detailed disclosure of the preferred embodiments ofmy invention so that those skilled in the art may practice the same, Icontemplate that variations may be made Without departing from the scopeof the invention as defined in the appended claims.

I claim:

1. In the manufacture of heat and pressure consolidated wood fiberhardboard, wherein wood has been educed substantially to fibers and saidfibers have been subjected to an aqueous digestion wherein a substantialquantity of water soluble derivatives thereof are produced, the furthersteps comprising wet felting said fibers to form a porous wood-fiber matcontaining a substantial proportion of said water soluble derivatives,drying the mat to a moisture content of less than 10%, effecting saiddrying of said mat in a manner whereby a substantial proportion of saidwater soluble derivatives are caused to move to a thin outer skin oneach face of said mat, heating said thin outer skin of each facesufliciently to cause said water solubles adjacent said faces to beconverted to a substantially water insoluble state without substantiallyheating the mat inward from said skin, without consolidating said mat toa specific gravity greater than approximately 0.2 less than the specificgravity of the ultimate hardboard, and then consolidating said mat bysuitable heat and pressure.

2. The method of claim 1 wherein said thin outer skin is heated to atemperature in excess of 400 F.

3. The method of claim 2 wherein said heating is for a period in theorder of one second.

4. The method of claim 2 wherein said heating is performed by contactingsaid skin with a heated external element.

5. The method of claim 4 wherein a plurality of said contacting stepsare included.

6. The method of claim 2 wherein said heating is accompanied with acompression of said mat without producing a permanent densification ofsaid mat to a. specific gravity above approximately 0.2 less than theultimate density of hardboard to be produced.

7. The method of claim 6 wherein said heating and compression isperformed by heated rollers.

8. The method of claim 6 wherein said heating and compression isperformed by heated platens.

9. The method of claim 2 wherein said heating is performed by radiationof heat from a heating source.

10. The method of claim 2 wherein said heating is performed byconvection of heat in a hot air oven.

References Cited in the file of this patent UNITED STATES PATENTS2,140,189 Mason Dec. 13, 1938 2,348,081 Linzell May 2, 1944 2,724,307Stewart Nov. 22, 1955

1. IN THE MANUFACTURE OF HEAT AND PRESSURE CONSOLIDATED WOOD FIBERHARDBOARD, WHEREIN WOOD HAS BEEN REDUCED SUBSTANTIALLY TO FIBERS ANDSAID FIBERS HAVE BEEN SUBJECTED TO AN AQUEOUS DIGESTION WHEREIN ASUBSTANTIAL QUANTITY OF WATER SOLUBLE DERIVATIVES THEREOF ARE PRODUCED,THE FURTHER STEPS COMPRISING WET FELTING SAID FIBERS TO FORM A POROUSWOOD-FIBER MAT CONTAINING A SUBSTANTIAL PROPORTION OF SAID WATER SOLUBLEDERIVATIVES, DRYING THE MAT TO A MOISTURE CONTENT OF LESS THAN 10%,EFFECTING SAID DRYING OF SAID MAT IN A MANNER WHEREBY A SUBSTANTIALPROPORTION OF SAID WATER SOLUBLE DERIVATIVES ARE CAUSED TO MOVE TO ATHIN OUTER SKIN OF EACH FACE OF SAID MAT, HEATING SAID THIN OUTER SKINOF EACH FACE SUFFICIENTLY TO CAUSE SAID WATER SOLUBLES ADJACENT SAIDFACES TO BE CONVERTED TO A SUBSTANTIALLY WATER INSOLUBLE STATE WITHOUTSUBSTANTIALLY HEATING THE MAT INWARD FROM SAID SKIN, WITHOUTCONSOLIDATING SAID MAT TO A SPECIFIC GRAVITY GREATER THAN APPROXIMATELY0.2 LESS THAN THE SPECIFIC GRAVITY OF THE ULTIMATE HARDBOARD, AND THENCONSOLIDATING SAID MAT BY SUITABLE HEAT AND PRESSURE.